Over the last years mankind has become increasingly aware of the environmental consequences of the methods that are and have been used for the disposal of waste materials. Particular attention has thus been paid to the pollution of surface waters, such as rivers (especially the river Rhine), and lakes, such as the Great Lakes in the USA, and also of seas and oceans, where pollution by for instance oil is a constantly recurring problem. More recently attention has been drawn to the pollution of soil. Numerous examples are known, where industrial waste has been deposited by burying metal drums containing the waste material, or where the waste has been led directly into nature in various ways. These acts have resulted in considerable cleansing operations. Concurrent with pollution due to "sins of the fathers" accidents occur despite increasingly sophisticated precautions where undesirable, environment-destroying, poisonous materials escape into nature. Examples of such accidents include oil-pollution of coastal areas, for instance after ship collisions or loss of ship, or of other areas, for instance after traffic accidents. Various attempts have been made to control environmental pollution, but none of the methods suggested has resulted in a solution of the problems sufficiently attractive with respect to costs involved and the possibility of fast action.
Today several cases are known, where contaminated areas have been discovered, for instance, during the excavation of a building site. A common method used today to remove such contaminations comprises excavating the entire amount of contaminated soil and filling it into drums. These drums are sent to a specialized incinerator plant, such as Kommunekemi in Denmark. Here the entire material is incinerated under controlled conditions ensuring a high combustion temperature. It is necessary to operate with high combustion temperatures, since lower temperatures result in a generation of dioxines, which are generally known to be very toxic. This method is not very suitable as the work connected with excavating the soil, filling it into drums, transporting it to the plant, opening of the drums and filling the material into the incinerator plant is extensive and not at all safe. Transporting large amounts of material is very cost-intensive and the capacity of the incinerator plant has to be considerable. In practice it is almost impossible to return the material to its place of origin, partly because of the transport costs involved and partly because such a return would require a further technical treatment of the material subsequent to its incineration.
For minor contaminations a method is used, where the costs for transport and incineration can be reduced. In this method a controlled deposit is created in form of an excavation provided with a sealing membrane onto which the contaminated material is deposited. With such a deposit it is mandatory to collect and examine, and, if necessary, to treat the rainwater seeping through the material. The deposit has thus to be constantly monitored, often over a period covering many years.
Both above methods are cost intensive, since the amount of contaminated material to be excavated and transported is large. A further problem connected with the incineration is that the contaminated material is often very heterogenous and may result in heavy wear of the plant, for instance in case of brickwork in a rotor incinerator. Another problem is how to control the incineration of the heterogenous waste, which often comprises for instance bituminous residues, phenolic compounds and frequently large amounts of polyaromatic hydrocarbons. To ensure that no poisonous materials, such as dioxines, escape together with the smoke, the incineration has to be made under very stationary conditions. It is, however, difficult and expensive to ensure a stationary incineration of the heterogenous materials in question.
EP Publication No. 0.161.698 Al discloses a method for purifying soil, such as from petroleum products, by extraction in a scrubber by means of an aqueous liquid of a pH value adapted to the contaminant, whereupon the extractant and the contaminants are separated. In this method the coarse materials are first separated from the soil by means of a sieve unit. This step implies that a portion of the contaminants is removed together with the coarse material and is thus not eliminated. Then the soil and the liquid are mixed. Subsequently the phases are separated and the liquid used is purified by floatation and is re-used. This method requires intricate and expensive equipment, since it requires a rugged device for mixing soil and water as well as a floatation tank. From the results stated in the publication it is moreover apparent that only a partial decontamination is obtained.
EP Publication No. 0.059.020 Al discloses a method for extracting contaminants from soil by means of water, alcohols or petroleum ether. The method is carried out by introducing the extractant directly into the contaminated soil through tubes extending into the soil. If necessary the contaminated area is first screened off from the surface layer and down to a water-impervious layer. Despite the screening there is a considerable risk of further contamination by spreading the contaminants and the extractant. It is thus mandatory to take extensive precautionary measures. Moreover comprehensive prior investigations concerning the type of contamination and soil have to be undertaken. Finally the method requires large amounts of extractant.
WO Publication No. 86/03134 discloses a method for separating bitumen from broken road surface materials by extraction, preferably with toluene in several steps followed by distillation and recovery of the solvent by condensation. The method is directed to the re-use of asphalt, which means that not all types of compounds usually occuring in connection with contaminated soil are removed. Bituminous material occurs in solid form. It is thus necessary to employ a comparatively sophisticated assembly for carrying out said method using high-pressure steam as well as vacuum together with heat.
GB Patent Application No. 2.022.444 discloses a process for the treatment of oil-containing waste by means of countercurrent extraction in several compartments using a solvent such as kerosene, i.e. a solvent ligther than water. A rotating container is used, said container comprising blades or buckets in each compartment to ensure contact between the waste material and the extractant. This patent application discloses a mechanically complicated process, where the material has to be present in form of an aqueous suspension. The process is applicable for aqueous waste materials and cannot thus be used for purifying contaminated soil materials.
EP Publication No. 0.172.056 Al discloses a method for washing out oil-containing sand or gravel, where the washing is performed with a solvent composition comprising kerosene and surfactants. Subsequent to the washing the solvent and the oily products are separated. This separation is performed by means of intricate equipment comprising hydrocyclones and flocculation units. The method is unsuitable for removing contaminants of a high content of polymeric compounds.
WO Publication No. 82/04440 discloses a method of separating oil or bitumen from oil sands or tar sands, where a chlorinated hydrocarbon, such as methylene chloride, is used as solvent and a liquid, such as water or alcohol, is used for subsequent treatment. The subsequent treatment allows an easy separation of the oily solution from the sand due to the influence of the solvent on the surfaces of the sand grains. It is also possible to use water first and then the extractant, but it is preferred to use water last. When carrying out the method complicated equipment is used, since the sandy material usually has to be ground and mixed mechanically with the liquids used. Moreover centrifugation or fluidisation are used to recover the solvent.
EP Publication No. 0.070.593 Al discloses a method for purifying soil by means of a heating medium, especially steam. The method is carried out in situ by injecting the steam through lances extending into the soil. The volatile contaminants driven out of the soil are collected by means of a vacuum clock. Treating the contaminated soil by heating to high temperatures involves a high energy consumption. Another problem is the risk of the contaminants spreading to the adjacent areas despite the use of the vacuum clock. Moreover the method is difficult to control. Finally said method does not remove non- or low-volatile or solid products, such as asphalt, tar and the like.
Different methods for the recovery of oil, bitumen or bituminous oil from oil sand, tar sand or the like by extraction are also known. Such methods are for instance disclosed in U.S. Pat. Nos. 4.424.112, 4.424.113, 4.532.024, 2.596.793, 4.387.016, 3.475.318 and 1.514.113. These methods are, however, directed to recovering the desired materials and are thus not suitable for removing a broad spectrum of contaminants. They are neither intended nor suitable for removing the last residues of the materials/contaminants, since this would render such a process too expensive.
It is also known to remove contaminants by only heating soil. This method does, however, not remove all contaminants. When the material contains for instance high-molecular, chlorinated compounds, such as PCB, chlorinated paraffines or the like, a moderate heating, i.e. a heating to below 1200.degree. C., can cause a partial decomposition of such compounds. The resulting decomposed products can have a negative influence on the environment, as they may be highly toxic, and may exceed the negative environmental impact of the original contaminants.
It is furthermore known to remove contaminants by biological means, which is, however, a very time-consuming process. Moreover, biological methods can only be used in case of slightly oil-contaminated materials. In case of contamination by heavy metals and other substances that can accumulate in an organism, biological cleansing will result in a later secretion of the heavy metals and other substances by the organism involved. As a result the contaminants are only transferred to an organism rather than being completely removed. Biological methods for fighting pollution demand considerable space and depend on various variables, such as the type of pollution as well as temperature and humidity conditions.